We have seen in the previous section how to render, with delays and reverbs, the distance of an instrument on a virtual stage.

The other space dimension is the left-right position: that's what stereo is made for.

Letís review the basic rules of stereo:

Rule 1: When two identical signals are sent to the speakers, these two audio sources combine perfectly and the sound comes from the center: we get mono from a stereo system.

Rule 2: When two identical signals are sent to the speakers, with a phase inversion, the sound is cancelled at the center, and the sound seems to come from two separate sources. Thatís what we get when the connection of one speaker is inverted on a hi-fi set.

Rule 3: When two identical signals are sent to the speakers, one of them being stronger, the sound comes from the strongest side: thatís panning.

Thatís all about stereo, as long as we deal with plain signals: no echo, no delay, no reverb. We can control the sound along the horizontal, left/right axis. We can get a fixed position by playing with the balance (panning control), split the sound in two with a fixed phase inversion, or modulate the panning to obtain a rotating autopan effect.

But we can go further, and use short delays to get more: create a wall of sound effect, as if the instrument was located not at a given point, but all along the left-right axis: that's the stereo chorus.

Ask two guys to climb on stage, with a few meters between them. Let them play exactly the same guitar part. It will sound somehow like a 12-strings guitar, and it will actually sound like two guitars, because they will not play strictly the same thing.

Instead of two guys, just keep one, and connect his guitar to two amps, a few meters between each other. It wonít sound like two guitars: it will sound like one guitar, and you will hear it at the center: remember stereo, Rule 1.

Letís set a 20ms delay between the two amps, which corresponds to a distance of 6m or so. We'll get a weird stereo effect, with a first feeling of space. But it's not sufficient to provide the feeling that two guitarists are playing along. It will mostly sound like someone playing between the walls of his bathroom, not what we call a wall of sound...

But if you 1) set a 20ms delay between the two amps, 2) modulate slowly this delay between, letís say 17ms and 23ms, it will sound almost like two guitar players! Thatís what a stereo chorus does. The sound doesnít come from the center of the stage: it comes from the complete stage width, as if two guitars were playing along.

The principle of a chorus (doubler) is to apply a short delay, between 20 and 50ms, and to modulate slightly this delay, to avoid hearing two clearly distinct sounds. The listener has thus the feeling of having two instruments merging their sound, but one cannot say where the sound comes from. The final result is a wide stereo image, as if the instrument was not located at a given point, but all along the stage width.

The 20ms to 50ms choice is not an innocent choice. Each time period can be associated to a frequency, by the simple formula F = 1/T. The human perception ranges from, roughly, 20Hz to 20kHz, and the really audible sounds start around 40Hz.

The period of a 20Hz sound is 1/20=0.05s, i.e. 50ms. The period of a 50Hz sound (approximately the lowest note of a bass guitar) is 20ms. In other terms:

A longer delay would make the chorus sound like a very short echo.

A shorter delay would cause sound interferences that would alter the tone of the instrument (thatís the comb filter effect used in the flangers).

I will not address flanging, phasing and the comb-filter principle here. It would be very interesting, but there is actually no effect of that kind in the Rockman line.

Stereo chorus, sometimes called doubling, has played a significant role in the history of SR&D and Rockman.

The Rockman Stereo Chorus, developed on the basis of Tom Scholz original doubler, is the main device used to convert a mono signal into a wide stereo sound, without altering the original instrument tone. The ďlong chorusĒ switch available in the Rockman chorus rack units changes the average delay from 20 to 50ms, thus allowing different sonic atmosphere and a deeper stereo effect.